In recent years, from the standpoint of reduction of environmental burdens, there has been a need for a clean liquid fuel which has a low content of sulfur and aromatic hydrocarbons and is compatible with the environment. Thus, in the petroleum industry, a Fisher-Tropsch synthesis method (hereinafter abbreviated as “FT synthesis method) using carbon monoxide and hydrogen as raw materials has been investigated as a method of manufacturing a clean fuel. The FT synthesis method has high expectations since it can manufacture a liquid fuel base stock which has an abundance of paraffin and which does not contain sulfur, for example, a diesel fuel base stock. For example, Patent Document 1 discloses a fuel oil compatible with the environment.
Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2004-323626
A synthetic oil obtained by the FT synthesis method (hereinafter may be referred to as “FT synthetic oil”) has a broad carbon number distribution. From the FT synthetic oil, it is possible to obtain, for example, an FT naphtha fraction containing a number of hydrocarbons having a boiling point of less than 150° C., an FT middle fraction containing a number of hydrocarbons having a boiling point of 150° C. to 360° C.; and an FT wax fraction heavier than the FT middle fraction.